1. Field of the Invention
The present invention relates to a damping force adjusting type hydraulic shock absorber mounted on a suspension system and the like of a vehicle such as an automobile.
2. Description of the Related Art
A hydraulic shock absorber mounted on a suspension system of a vehicle such as an automobile typically comprises: a cylinder containing hydraulic fluid sealed therein; a piston slidably fitted in the cylinder for dividing the interior of the cylinder into a first chamber and a second chamber; a piston rod including one end connected to the piston and an opposite end extending through the first chamber out of the cylinder; and a hydraulic fluid passage having a damping force generating mechanism (orifice, disc valve, and so on) for communicating between the first and second chambers. The movement of the piston associated with the extension and contraction of the piston rod allows the hydraulic fluid to flow in the hydraulic fluid passage. The damping force generating mechanism controls the flow of the hydraulic fluid in the hydraulic fluid passage to generate a damping force against the extension and contraction of the piston rod.
Long term use of the hydraulic shock absorber results in wear of sliding parts such as the piston, the piston rod, and so on with the result that metal powder created by the wear enters the hydraulic fluid within the cylinder. Long term use also results in other foreign matter entering the hydraulic fluid within the cylinder from the outside. Such metal powder and other foreign matter hastens the wear of the sliding parts and may cause mechanical failure of the damping force generating mechanism. Therefore, a hydraulic shock absorber is provided in which a filter is located in the hydraulic fluid passage to filter foreign matter present in the hydraulic fluid (Japanese Patent Public Disclosure No. 2-31037).
However, a hydraulic shock absorber in which such a filter is provided only in the hydraulic fluid passage has the following defects. Namely, if the filter becomes clogged for some reason, the flow resistance of the hydraulic fluid passage suddenly increases to excessively increase the damping force of the hydraulic shock absorber, with a resulting deterioration in vehicle handling. In addition, it may excessively increase the pressure of the hydraulic fluid upstream of the filter and damage the filter.
Since the filter consists of a fine mesh or a porous material such as a sintered metal and the like, the flow resistance of the filter itself is susceptible to the viscosity of the hydraulic fluid. Thus, when the viscosity of the hydraulic fluid is raised at low temperature, the flow resistance of the filter increases, thereby excessively increasing the damping force of the hydraulic shock absorber with a resulting deterioration in vehicle handling. Such an increase in damping force is restrained to some extent by sufficiently increasing the effective area of the filter. However, due to space construction, the advantages of such a construction are limited.